Method for controlling driving stability

ABSTRACT

The present invention relates to a method for controlling driving stability of a vehicle wherein a number of input variables are used to control the driving speed of the vehicle to a limit speed. In order to prevent unstable driving conditions in a pre-controlled manner, a model-based stable limit speed of the vehicle is determined on the basis of measured variables that are detected by means of detection devices and represent the current steering angle and the current lateral acceleration, by including other quantities of the vehicle and/or the driving situation, and the driving speed is compared to the model-based stable limit speed of the vehicle, and based on the comparison result, the driving speed is adapted to the limit speed when the driving speed exceeds the limit speed.

This application is the U.S. national phase of international applicationPCT/EP02/06343 filed Jun. 10, 2002, which designated the U.S. and whichclaims the benefit of priority of German Patent Application Number 10128 357.1 filed Jun. 13, 2001. The contents of each of the aforementioneddocuments are incorporated herein in their entirety.

TECHNICAL FIELD

The present invention generally relates to a method for controllingdriving stability of a vehicle and more particularly relates to a methodfor limiting vehicle speed.

BACKGROUND OF THE INVENTION

Vehicle instability is likely to occur in defined driving situationsespecially when the vehicle speed is not adapted to vehicle or roadconditions. Various driving stability control systems have become knownin the art that aim at automatically counteracting vehicle instability.

SUMMARY OF THE INVENTION

There are basically five principles of influencing the drivingperformance of a vehicle by means of predeterminable pressures or brakeforces in or at individual wheel brakes and by means of interventioninto the engine management of the driving engine. These principles arebrake slip control (ABS) intended to prevent individual wheels fromlocking during a braking operation, traction slip control (TSC)preventing the driven wheels from spinning, electronic brake forcedistribution (EBV) controlling the ratio of brake forces between frontand rear axle of the vehicle, anti rollover braking (ARB) preventingrollover of the vehicle about its longitudinal axis, as well as yawtorque control (ESP) ensuring stable driving conditions when the vehicleyaws about its vertical axis.

Hence, the term ‘vehicle’ in this context implies a motor vehicle withfour wheels, which is equipped with a hydraulic, electrohydraulic orelectromechanical brake system. The driver is able to develop brakepressure in the hydraulic brake system by means of a pedal-operatedmaster cylinder, while the electrohydraulic and electromechanical brakesystems develop a brake force responsive to the sensed braking demand ofthe driver.

Further, the vehicle is equipped with a thermodynamic or electricdriving system applying traction torque depending on the driver's demandto at least one wheel of the vehicle by way of the drive train.

To sense driving-dynamics condition[s], there are four rotational speedsensors, one per wheel, i.e. one yaw velocity sensor, one lateralacceleration sensor and at least one pressure sensor for the brakepressure generated by the brake pedal. Instead of the pressure sensor, apedal-travel or pedal-force sensor may also be used if the auxiliarypressure source is so arranged that brake pressure built up by thedriver cannot be distinguished from the brake pressure of the auxiliarypressure source. The driving torque currently generated by the drivingsystem and the torque the driver demands are determined in addition.These variables may also be variables that are indirectly determined,e.g. derived from engine performance characteristics.

The driving performance of a vehicle is influenced in a drivingstability control operation so that the vehicle is better to master forthe driver in critical situations. A critical situation in this respectis an unstable driving condition when the vehicle will not follow theinstructions of the driver in the extreme case. Thus, the function ofdriving stability control in such situations is to impart to the vehiclethe vehicle performance the driver requests, within physical limits.While longitudinal slip of the tires on the roadway is significant infirst place for brake slip control, traction slip control and electronicbrake force distribution, further variables are included in yaw torquecontrol (YTC), for example, the yaw rate and tire slip angle velocity.

Mostly, the cause of the critical situation is a generally too highdriving speed for a given driving situation. The given driving situationis characterized by a predefined desired curve radius and by thecoefficient of friction between tires and roadway.

It would be desirable to avoid unstable driving situations, which thedriver frequently cannot master, already as they develop so thatcritical driving situations are prevented from occurring.

An object of the present invention is to provide a method and a drivingdynamics control system for detecting imminent vehicle instability as aresult of too high driving speed and to counteract unstable drivingconditions already as they develop in a pre-controlling manner.

One possibility of reducing the speed of a vehicle in an understeer caseis described in EP 0 945 320 A1 wherein an adjustment value for thelongitudinal deceleration is calculated from the deviation of themeasured steering angle from a calculated steering angle. Apart fromother quantities, a maximum value of the lateral acceleration(saturation value) assessed in real time is taken into consideration inthe calculation of the steering angle.

This object is achieved by the present invention in that a model-basedstable limit speed of the vehicle is determined on the basis of measuredvariables that are detected by means of detection devices and representthe current steering angle and the current lateral acceleration, byincluding other quantities of the vehicle and/or driving situations,wherein the driving speed is compared to the model-based stable limitspeed of the vehicle, and based on the result of comparison, the drivingspeed is adapted to the limit speed when the driving speed exceeds thelimit speed.

A method of this type allows adapting the driving speed to themodel-based stable limit speed especially during cornering in order toavoid unstable driving conditions in a pre-controlled manner, therebypreventing that excessive vehicle acceleration will cause the vehiclespeed to exceed a maximum speed derived from the driving situation. Tothis end, the maximum speed is found out in a given driving situationpermitting to pass through this driving situation at the stabilitylimit. Further, it is found out whether there is a tendency of exceedingthis maximum speed and, thus, a tendency towards a subsequent unstabledriving behavior, and that in this case the vehicle is prevented fromexceeding the maximum speed by engine or brake intervention already whenstable driving performance prevails. The result is that a criticaldriving situation is either avoided already as it develops, orinstability is reduced to an extent enabling the driver to master thesituation. This counteracts imminent instability of the vehicle alreadyas it is produced so that unstable driving conditions in corneringmaneuvers can be avoided during which, for example, the yaw torquecontroller (ESP interventions) would have to intervene. In dependence onvehicle state variables and/or information about the driving performanceor driving situation, it can be determined how the stable limit speed ismodeled by varying the steering angle deviation tolerated.

Another objective of the invention is to design a generic drivingstability control system for a vehicle in such a way that drivingstability control is characterized by a steering angle detection devicefor determining a steering angle of the steering wheel, a lateralacceleration detection device for determining lateral acceleration, acomputing device for computing a stable limit speed from the determinedquantities steering angle of the steering wheel and lateral accelerationby including further quantities of the vehicle and/or drivingsituations, a speed detection device for detecting a current vehiclespeed and a speed control device controlling the vehicle speed to thecalculated stable limit speed as soon as the vehicle speed shows thetendency of exceeding the stable limit speed.

To enhance driving stability it is suitable that the stable limit speedis directly determined from quantities prevailing in an ESP controlledvehicle. Advantageously, the limit speed is directly determinedaccording to the recursive relation

$v_{limit} = \sqrt{\frac{l*a_{q}}{\delta - {\Delta\delta}_{tol} - {a_{q}*{EG}}}}$where

-   -   δ—steering angle of the steering wheel at the wheel    -   Δδ_(tol) tolerated steering angle deviation compared to the        linear case    -   l—distance of the vehicle axles (wheel base)    -   ν_(lim it) stable limit speed    -   EG—self-steering gradient of the vehicle    -   α_(q)—vehicle lateral acceleration.

Advantageously, the speed difference between driving speed and stablelimit speed is reduced by way of a brake and/or engine intervention, andit can be defined in dependence on vehicle state variables and/orinformation about the driving performance or the driving situation atwhat intensity interventions are executed.

It is favorable that the control of the driving speed to the limit speedis terminated when a driving speed desired by the driver is found outwhich lies below the stable limit speed.

According to another favorable embodiment, the lateral inclination ofthe vehicle or the roadway with respect to the horizontal plane isdetermined, and the tolerated steering angle deviation is computed independence on the determined lateral inclination.

It is expedient that the steering angle deviation is increased indriving situations with an increased traction requirement.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a vehicle having ESP control system,brake system, sensor system and communication means.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now referring to FIG. 1, the four wheels are designated by referencenumerals 15, 16, 20, 21. One wheel sensor 22 to 25 is provided on eachof the wheels 15, 16, 20, 21. The signals are sent to an electroniccontrol unit 28 determining the vehicle speed V_(Ref) from the wheelrotational speeds by way of predetermined criteria. Further, a yaw ratesensor 26, a lateral acceleration sensor 27, and a steering angle sensor29 are connected to the electronic control unit 28. Each wheeladditionally includes an individually controllable wheel brake 30 to 33.Said brakes are hydraulically operated and receive pressurized hydraulicfluid by way of hydraulic lines 34 to 37. Brake pressure is adjusted bymeans of a valve block 38, said valve block being actuated independentlyof the driver on command of electric signals that are generated in theelectronic control unit 28. The driver is able to introduce brakepressure into the hydraulic lines by way of a master cylinder actuatedby a brake pedal. Pressure sensors P that allow sensing the driver'sbraking demand are provided in the master cylinder or the hydrauliclines. The electronic control unit is connected to the engine controldevice by way of an interface (CAN).

By way of the ESP control system with brake system, sensor system andcommunication means including the components

-   -   Four wheel speed sensors    -   Pressure sensor (P)    -   Lateral acceleration sensor (LA)    -   Yaw rate sensor (YR)    -   Steering angle sensor (SWA)    -   individually actuatable wheel brakes    -   Hydraulic unit (HCU)    -   Electronic control unit (ECU)        it is possible to realize a forecast of a critical driving        situation and, preferably, avoid it without the use of        additional sensors. Based on the equation for the stationary        circular travel of the linear one-track model

$\overset{.}{\Psi} = \frac{\delta \cdot v}{l + {v^{2} \cdot {EG}}}$and the relation

$\overset{.}{\Psi} = \frac{a_{q}}{v}$where

-   {dot over (Ψ)}—yaw rate of the vehicle-   δ—steering angle of the front wheels-   ν—vehicle speed-   l—axle base-   EG—self-steering gradient of the vehicle-   α_(q)—lateral acceleration of the vehicle    it is possible to calculate the stable vehicle speed in a driving    situation by way of the equation

$v = {\sqrt{\frac{l \cdot a_{q}}{\delta - {a_{q} \cdot {EG}}}}.}$

Reaching of the stability limit announces itself by leaving the linearrange. Thus, the stable limit speed of a driving situation can bedetermined by taking into consideration a tolerated steering angledeviation compared to the linear performance.

$v_{limit} = \sqrt{\frac{l \cdot a_{q}}{\delta - {\Delta\delta}_{tol} - {a_{q} \cdot {EG}}}}$wherein

-   ν_(lim it)—stable limit speed-   Δδ_(tol)—tolerated steering angle deviation compared to the linear    case.

The higher the magnitude of tolerated steering angle deviation ischosen, the better the calculated limit speed and the stability reservewill be utilized.

Appropriately, the limit speed is calculated continuously, and it is setfor infinity should there be no real solution of the above equation forthe current input variables. With a stable driving style, the limitspeed will always lie above the actual vehicle speed. Only whenapproaching the stability limit will the limit speed be reached orexceeded. This may e.g. be caused by a sharper steering inwards, byvehicle acceleration, or by reducing the coefficient of friction betweentires and roadway and, thus, by reducing the lateral acceleration. It isof no significance, whether the vehicle understeers, oversteers ordrifts off laterally when the stability limit is exceeded.

According to the invention, execution of a brake or engine interventionwill counteract the exceeding of the stability limit as soon as themeasured vehicle speed has a tendency to exceeding the limit speed thatis calculated according to the above equation. The brake and/or engineintervention will now be carried out in such a way that the vehiclespeed is controlled to the limit speed. This maintains the vehicle inthe stable limit range. It is easy for the driver to master the vehicle.In addition, abrupt and disharmonious control interventions are avoided,exactly as they are produced by yaw torque control when the stabilitylimit is exceeded.

Brake and/or engine intervention is terminated as soon as the vehiclespeed desired by the driver lies considerably below the calculated limitspeed. This may e.g. be caused by steering outwards when leaving acurve, by vehicle deceleration or by increasing the coefficient offriction between tires and roadway and, thus, by increasing the lateralacceleration.

When the roadway has a lateral inclination, the lateral accelerationused to calculate the stable limit speed may be determined as too low aquantity. To prevent that the limit speed is calculated as a too lowquantity, it is suitable to appropriately determine the lateralinclination of the roadway and to increase the tolerated steering angledeviation in dependence on the lateral inclination.

In driving situations requiring increased traction, that means e.g. whendriving in sand or deep snow, great steering angles may temporarilyoccur without the vehicle moving at the stability limit. In addition,driving torque reductions are extremely disturbing in these situationsbecause they may cause the vehicle to dig into the ground and get stuck.These situations are characterized e.g. by wheel brake slip on at leastone front wheel with a positive driving torque and lacking brake torqueof the wheel brake. When this situation is detected, it is likewiseexpedient to increase the tolerated steering angle deviation.

1. Method for controlling driving stability of a vehicle, comprising:establishing by means of detection devices a current steering angle anda current lateral acceleration; determining a model-based stable limitspeed of the vehicle, wherein the model-based stable limit speed isdetermined based on the current steering angle and the current lateralacceleration wherein a driving speed is compared to the model-basedstable limit speed of the vehicle, and based on a result of comparison,the driving speed is adapted to the limit speed when the driving speedexceeds the limit speed; and wherein the model-based stable limit speedis determined according to the recursive relation$v_{limit} = \sqrt{\frac{l*a_{q}}{\delta - {\Delta\delta}_{tol} - {a_{q}*{EG}}}}$where δ is a steering angle of the steering wheel at the front wheels ξis wheel base EG is a self-steering gradient of the vehicle α_(G) is avehicle lateral acceleration υ_(lim it) is a stable limit speed Δδ_(tol)is a tolerated steering angle deviation compared to the linear case. 2.Method as claimed in claim 1, further including the step of: conductinga brake intervention or engine intervention according to the differencebetween the driving speed and the limit speed.
 3. Method as claimed inclaim 1, wherein the tolerated steering angle deviation is determinedaccording to the lateral acceleration and the driving speed.
 4. Methodas claimed in claim 1, further including the step of: terminating thecontrol of the driving speed towards the limit speed when a drivingspeed desired by the driver is determined which lies below themodel-based stable limit speed.
 5. Method as claimed in claim 1, furtherincluding the step of: determining the lateral inclination of thevehicle or the roadway with respect to the horizontal plane, andcomputing the tolerated steering angle deviation in dependence on thedetermined lateral inclination.
 6. Method as claimed in claim 5, furtherincluding the step of: increasing the steering angle deviation toleratedin driving situations with an increased traction requirement.
 7. Drivingstability control system for a vehicle, comprising: a control device forcontrolling the vehicle speed; a steering angle detection device fordetecting a steering angle of a steering wheel of the vehicle; a lateralacceleration detection device for determining a lateral acceleration ofthe vehicle; a computing device for computing a stable limit speed fromthe determined quantities steering angle of the steering wheel andlateral acceleration of the vehicle; a speed detection device fordetecting a current vehicle speed, wherein said control device controlsthe vehicle speed to a calculated stable limit speed whenever thevehicle speed exceeds the stable limit speed; and wherein the computingdevise determines the stable limit speed according to the recursiverelation$v_{limit} = \sqrt{\frac{l*a_{q}}{\delta - {\Delta\delta}_{tol} - {a_{q}*{EG}}}}$where δ is a steering angle of the steering wheel at the front wheels ξis wheel base EG is a self-steering gradient of the vehicle α_(q) is avehicle lateral acceleration υ_(lim it) is a stable limit speed Δδ_(tol)is a tolerated steering angle deviation compared to the linear case. 8.Driving stability control as claimed in claim 7, wherein the controldevice controls the vehicle speed so that it does not exceed the stablelimit speed by means of brake intervention or by intervention into theengine driving torque.
 9. Driving stability control as claimed in claim8, wherein the brake or engine intervention is terminated as soon as thevehicle speed desired by the driver lies below the stable limit speed.10. Driving stability control as claimed in claim 7, further including alateral inclination detection device for determining a lateralinclination of the roadway which information is in turn used by thecomputing device to alter the steering angle deviation tolerated independence on the determined lateral inclination.
 11. Driving stabilitycontrol as claimed in claim 7, further including a determining unit fordetermining the traction of the vehicle wherein the computing deviceuses this information to alter the steering angle deviation tolerated.